Nature of segregation of reactants in diffusion controlled A+B reactions: Role of mobility in forming compact clusters

We investigate the A+B=0 bimolecular chemical reaction taking place in low-dimensional spaces when the mobilities of the two reacting species are not equal. While the case of different reactant mobilities has been previously reported as not affecting the scaling of the reactant densities with time, but only the pre-exponential factor, the mechanism for this had not been explained before. By using Monte-Carlo simulations we show that the nature of segregation is very different when compared to the normal case of equal reactant mobilities. The clusters of the mobile species are statistically homogeneous and randomly distributed in space, but the clusters of the less mobile species are much more compact and restricted in space. Due to the asymmetric mobilities, the initial symmetric random density fluctuations in time turn into asymmetric density fluctuations. We explain this trend by calculating the correlation functions for the positions of particles for the several different cases

Selective Benzyl Alcohol Oxidation over Pd Catalysts

In the last decades, the selective liquid phase oxidation of alcohols to the corresponding carbonyl compounds has been a subject of growing interest. Research has focused on green methods that use "clean" oxidants such as O-2 in combination with supported metal nanoparticles as the catalyst. Among the alcohols, benzyl alcohol is one of the most studied substrates. Indeed, benzyl alcohol can be converted to benzaldehyde, largely for use in the pharmaceutical and agricultural industries. This conversion serves as model reaction in testing new potential catalysts, that can then be applied to other systems. Pd based catalysts have been extensively studied as active catalytic metals for alcohol oxidation for their high activity and selectivity to the corresponding aldehyde. Several catalytic materials obtained by careful control of the morphology of Pd nanoparticles, (including bimetallic systems) and by tuning the support properties have been developed. Moreover, reaction conditions, including solvent, temperature, pressure and alcohol concentration have been investigated to tune the selectivity to the desired products. Different reaction mechanisms and microkinetic models have been proposed. The aim of this review is to provide a critical description of the recent advances on Pd catalyzed benzyl alcohol oxidation

Validation of a new three-dimensional imaging system using comparative craniofacial anthropometry

Abstract Background The aim of this study is to validate a new three-dimensional craniofacial stereophotogrammetry imaging system (3dMDface) through comparison with manual facial surface anthropometry. The null hypothesis was that there is no difference between craniofacial measurements using anthropometry vs. the 3dMDface system. Methods Facial images using the new 3dMDface system were taken from six randomly selected subjects, sitting in natural head position, on six separate occasions each 1 week apart, repeated twice at each sitting. Exclusion criteria were excess facial hair, facial piercings and undergoing current dentofacial treatment. 3dMDvultus software allowed facial landmarks to be marked and measurements recorded. The same measurements were taken using manual anthropometry, using soluble eyeliner to pinpoint landmarks, and sliding and spreading callipers and measuring tape to measure distances. The setting for the investigation was a dental teaching hospital and regional (secondary and tertiary care) cleft centre. The main outcome measure was comparison of the craniofacial measurements using the two aforementioned techniques. Results The results showed good agreement between craniofacial measurements using the 3dMDface system compared with manual anthropometry. For all measurements, except chin height and labial fissure width, there was a greater variability with the manual method compared to 3D assessment. Overall, there was a significantly greater variability in manual compared with 3D assessments (p < 0.02). Conclusions The 3dMDface system is validated for craniofacial measurements

Kinetic Evidence for a Non-Langmuir-Hinshelwood Surface Reaction: H/D Exchange over Pd Nanoparticles and Pd(111)

The mechanism of hydrogen recombination on a Pd(111) single crystal and well-defined Pd nanoparticles is studied using pulsed multi-molecular beam techniques and the H2/D2 isotope exchange reaction. The focus of this study is to obtain a microscopic understanding of the role of subsurface hydrogen in enhancing the associative desorption of molecular hydrogen. HD production from H2 and D2 over Pd is investigated using pulsed molecular beams, and the temperature dependence and reaction orders are obtained for the rate of HD production under various reaction conditions designed to modulate the amount of subsurface hydrogen present. The experimental data are compared to the results of kinetic modeling based on different mechanisms for hydrogen recombination. We found that under conditions where virtually no subsurface hydrogen species are present, the HD formation rate can be described exceptionally well by a classic Langmuir–Hinshelwood model. However, this model completely fails to reproduce the experimentally observed high HD formation rates and the reaction orders under reaction conditions where subsurface hydrogen is present. To analyze this phenomenon, we develop two kinetic models that account for the role of subsurface hydrogen. First, we investigate the possibility of a change in the reaction mechanism, where recombination of one subsurface and one surface hydrogen species (known as a breakthrough mechanism) becomes dominant when subsurface hydrogen is present. Second, we investigate the possibility of the modified Langmuir–Hinshelwood mechanism with subsurface hydrogen lowering the activation energy for recombination of two hydrogen species adsorbed on the surface. We show that the experimental reaction kinetics can be well described by both kinetic models based on non-Langmuir–Hinshelwood-type mechanisms

Microkinetic Modeling of Benzyl Alcohol Oxidation on Carbon Supported Pd and AuPd Nanoparticles

Experiments were conducted on the liquid-phase oxidation of benzyl alcohol over Pd and AuPd nanoparticles, with the aim of determining the reaction mechanism. It was determined that there are two primary reaction paths: A) an alkoxy pathway leading to toluene, benzaldehyde, and benzyl ether, and B) a carbonyloxyl pathway (\u201cneutral carboxylate\u201d) leading to benzoic acid, benzene, and benzyl benzoate. Therefore, microkinetic modelling using the obtained mechanism with surface intermediates was capable of producing all experimentally observed trends with mostly quantitative agreement. 1. Scope Metal particles can directly catalyze the liquid phase oxidation of alcohols using molecular oxygen as the oxidant.1 In particular, benzyl alcohol oxidation to benzaldehyde is of practical use for pharmaceutical, perfume, dye, and agricultural industries.2 Depending on the reaction conditions (temperature, solvent, oxygen pressure), many side products including benzene, toluene, benzoic acid, benzyl benzoate, and benzyl ether have been reported to be formed in addition to the main product, benzaldehyde. Tentative mechanisms were proposed. However, a detailed and complete mechanism of Pd (AuPd) catalyzed benzyl alcohol oxidation in organic solvent has not yet been proposed. In this study, we have performed experiments in which the temperature, gas-phase oxygen pressure, and initial concentration of the benzyl alcohol were varied to elucidate the mechanism.3 Furthermore microkinetic modeling (simulation and fitting) of the reaction were performed.4,5 The objectives of this microkinetic modeling are threefold: 1) to provide additional evidence for the mechanism used by verifying that kinetic modeling with this mechanism can reproduce the kinetic behavior observed experimentally (absolute quantities produced and selectivity trends), 2) to identify which reactions are the most kinetically significant, and 3) to extract kinetic parameters for use in future modeling/studies. 2. Results and discussion The liquid-phase oxidation of benzyl alcohol over Pd and AuPd nanoparticles supported on a cwtiavsa tpeedr fcoarrmboend. xEyxlpeenreim aesn tsth ew esroel vpeenrtf oarnmde dc oinnt ian uboautsc hg arse acptuorrg iwngit ho fp atrhae- hvaeraidesdp:a cthee. iTnhiteia l foblelnozwyiln ga lcoexhpoel ricmonencetanlt raptiaorna,m tehteer so xwygeerne pFarortmia lt rpernedsss uinre t hine cthoen cheenatdrastpiaocne p, raonfdil etsh ea nrde aicnttoerg rtaetmedp eprraotduurec.t ion of each product, it was determined that Scheme 1. Proposed mechanism for benzyl alcohol oxidation.3 tehtheerer, aarned t wBo) ap rciamrbaroyn yreloaxcytilo np apthawthasy: A(\u201c)n eaunt raallk ocxaryb opxaythlawtea\u201dy) lleeaaddiinngg ttoo tboeluneznoeic, baceindz,a bldeenhzyednee,, aanndd bbeennzzyyll bTehnez omatiec r(oSkcihneemtice 1m).o deling in this work was able to reproduce the selectivities and trends observed for the production of both the main product (benzaldehyde) and the byproducts (benzene, toluene, benzoic acid, benzyl benzoate, and benzyl ether). The present study suggests that the most important activation energies are those of k2, k5, and k6 (Scheme 1), which we estimate as Ea2=57.9 kJmol-1, Ea5=129 kJmol-1, and Ea6= 175 kJmol- 1 corresponding to alcohol dissociation, alkyl hydrogenation, and reaction of alkyl species with alkoxy species. Under the same reaction conditions, AuPd/C has a lower activity compared to Pd/C and shows a different product distribution with less formation of products from the \u201ccarbonyloxyl\u201d pathway (benzene, benzoic acid, benzoate). It was found that the selectivity changes can be explained by this change in k1, which corresponds to oxygen adsorption (Figure 1). 3. Conclusions Quantitative kinetic analysis of benzyl alcohol oxidation over carbon-supported Pd and AuPd nanoparticles has enabled us to elucidate the reaction mechanism. The proposed mechanism suggests that the selectivity is influenced by not only temperature and the coverage of the reactants, but also by the side reaction of oxygen scavenging surface hydrogen. Additional insights on how the rate constants affect the production of each product (and thus selectivities) were gained from the analytical equations that were derived from the microkinetic model. References 1. M. Besson, P. Gallezot, Catal. Today 2000, 57, 127-141. 2. T. Mallat, A. Baiker, Chem Rev 2004, 104, 3037-3058. 3. A. Savara, C. E. Chan-Thaw, I. Rossetti, A. Villa, L. Prati, ChemCatChem 2014, 6, 3464\u20133473. 4. A. Savara, I. Rossetti, C. E. Chan-Thaw, L. Prati, A. Villa, ChemCatChem 2016, 8, 2482 \u20132491. 5. A. Savara, C. E. Chan-Thaw, J. E. Sutton, D. Wang, L. Prati, A. Villa, ChemCatChem DOI: 10.1002/cctc.201601295

Temperature dependence of the 2-butene hydrogenation over supported Pd nanoparticles and Pd(111)

The activity and induction times for 2-butene hydrogenation have been investigated over a Pd(1 1 1) single crystal surface and model Pd nanoparticles supported on Fe3O4/Pt(1 1 1) by isothermal pulsed molecular beam experiments, in the temperature range of 220–340 K. C-modification of supported Pd particles induced persistent hydrogenation activity at low temperatures (220–260 K). C-modification of the Pd(1 1 1) surface, in contrast, did not result in significant reactivity changes. At low temperatures (220–260 K), hydrogenation activity was only maintained over the C-modified Pd particles, while at temperatures (≥280 K) persistent hydrogenation was observed over all Pd catalysts at comparable rates. Two principal reaction mechanisms are discussed that could be responsible for the observed hydrogenation activity at different Pd surfaces. We show that on Pd nanoparticles, the reaction mechanism involving subsurface hydrogen species plays an important role under all investigated conditions. This subsurface-related reaction pathway relies on an effective replenishment of the subsurface hydrogen reservoir, which is affected by the presence of strongly adsorbed hydrocarbon species that are formed in the induction period. We discuss the correlation between the induction times and the hydrogenation activity of different Pd surfaces

Interaction of Isophorone with Pd 111 A Combination of Infrared Reflection Absorption Spectroscopy, Near Edge X ray Absorption Fine Structure, and Density Functional Theory Studies

Atomistic level understanding of interaction of amp; 945;, amp; 946; unsaturated carbonyls with late transition metals is a key prerequisite for rational design of new catalytic materials with the desired selectivity toward C C or C O bond hydrogenation. The interaction of this class of compounds with transition metals was investigated on amp; 945;, amp; 946; unsaturated ketone isophorone on Pd 111 as a prototypical system. In this study, infrared reflection absorption spectroscopy IRAS , near edge X ray absorption fine structure NEXAFS experiments, and density functional theory calculations including van der Waals interactions DFT vdW were combined to obtain detailed information on the binding of isophorone to palladium at different coverages and on the effect of preadsorbed hydrogen on the binding and adsorption geometry. According to these experimental observations and the results of theoretical calculations, isophorone adsorbs on Pd 111 in a flatlying geometry at low coverages. With increasing coverage, both C C and C O bonds of isophorone tilt with respect to the surface plane. The tilting is considerably more pronounced for the C C bond on the pristine Pd 111 surface, indicating a prominent perturbation and structural distortion of the conjugated amp; 960; system upon interaction with Pd. Preadsorbed hydrogen leads to higher tilting angles of both amp; 960; bonds, which points to much weaker interaction of isophorone with hydrogen precovered Pd and suggests the conservation of the in plane geometry of the conjugated amp; 960; system. The results of the DFT vdW calculations provide further insights into the perturbation of the molecular structure of isophorone on Pd 11